At some point over the last few weeks the one millionth .co domain was registered, approximately ten months after the domain names became generally available.

That’s pretty good going (better than I expected) when compared to other large-scale TLD launches, such as .mobi, which took almost five years to hit the same milestone.

Registry .CO Internet has been marketing .co domains hard for the last 12 months, particularly in California, where it is focused on attracting Silicon Valley entrepreneurs.

To turn the one million milestone into a marketing event, the company has also released some customer endorsement videos at a new site, UnderTheBulb.co (which it’s advertising in my sidebar).

Whether the rapid growth is sustainable is now the question. The one-year anniversary of .co’s launch is coming up in late July, and we might expect a number of early speculative registrations to expire.

But .CO Internet seems confident that it won’t see much of a blip, as its numbers suggest that the vast majority of its registrants – reportedly north of 80% – own fewer than 10 domains each.

Chief executive Juan Calle joked that the company had considered a marketing campaign to coincide with the anniversary, with the slogan “Let ‘Em Drop”, to use a bit of reverse psychology on domainers.

“The registry growing fairly fast,” said Calle. “What happens is that if those domains get dropped they’ll get picked up by real users and businesses.”

The rapid growth is no doubt due in no small part to Go Daddy, which has been prominently featuring .co domains on its front page for months, and to promotional pricing.

Under the current promotion, .CO Internet has roughly halved its registry fee to about $9.50 for first-year registrations, which has translated into $11.99 domains at the checkout.

But Calle said the higher price of .co domains, usually around the $30 mark, does not deter its target customer base, which are business users rather than speculators.

“Pricing is secondary to marketing when it comes to the growth rate — when we do things like the Super Bowl, or when Overstock [which rebranded as o.co] runs their commercials for a week nationally,” he said.